Portable communication products require circuits that can perform well in a low power environment. A reduction of power supply voltages allows for fewer battery cells, reducing the size and weight of the portable equipment. However, the lower power constraint adversely affects the performance of the standard RF circuitry. Circuits are needed that can achieve the design goals for noise figure, linearity and power consumption for portable communications products.
In a two-way communication link, the transmit signal must be differentiated from the desired signal to be received. The received signal is passed through circuits that include low-noise amplifiers and mixers for down-converting the received signal in frequency from the Radio Frequency (RF) range to the Intermediate Frequency (IF) range. The mixer generates an output signal having a frequency that is the difference between the frequency of the received RF signal and the frequency of a local oscillator signal, thus converting the received signal to an IF signal.
The performance of filtering, frequency conversion and demodulation is sensitive to IC processing. The purpose of the receiver is to receive and process a signal while accounting for variations that affect frequency and tuning accuracy. An Automatic Frequency Control (AFC) signal is provided to adjust the receiver frequency demodulation process and accommodate operating power supply changes and component variations due to temperature and process. Typically, the AFC signal charges an external capacitor used by the tuning circuit for filtering. This capacitor value is large to prevent the oscillator from following the modulation signals and cancelling them. However, the large capacitance value limits the tuning speed of the AFC.
Accordingly, a need exists for a receiver that generates an AFC signal that has a fast operation. It would be a competitive advantage to provide a receiver that eliminated external components.